Several different groups of crustaceans are known as lobsters. When people talk about lobsters, most of the time they mean clawed lobsters, for example the genus Homarus. Clawed lobsters are not closely related to squat lobsters, spiny lobsters or slipper lobsters, which have no claws (chelae). The closest relatives of clawed lobsters are the reef lobsterEnoplometopus and the three groups of freshwater crayfish.

Smaller kinds of lobsters are sometimes called "lobsterettes". Lobsters are invertebrates. They have a hard exoskeleton (outer skeleton), which protects them. Similar to most arthropods, lobsters must shed (or lose) this skeleton before they can grow. This process of shedding is called moulting. When lobsters "moult", they are weak and easy to attack. Some lobster species' colors change when they are moulting.

Lobsters live between the shore and the edge of the continental shelf. They live on rocky, sandy, or muddy bottoms (of the water).They usually live alone in cracks or under rocks.

Lobsters usually eat live food, which includes fish, mollusks, other crustaceans, worms, and some plant life. Lobsters scavenge (eat dead plants and animals) when they need to. Lobsters also eat other lobsters (cannabalism) when they are not free; lobsters do not eat other lobsters in the oceans. People have found lobster skin in the stomachs of some lobsters. This is because lobsters eat the skin they shed when they moult. .[1] Lobsters grow all their lives. Some lobsters live for over 100 years ,[2] and some become very very big. The Guinness World Records says that the largest lobster was from Nova Scotia, Canada and weighed 20.14 kg (44.4 lb).

Similar to most arthropods, lobsters are "bilaterally symmetrical" - they are the same on the left and right sides. But lobsters who have claws often have claws of different sizes, for example the king crab. These unequal claws are for special actions. When a fisherman catches a lobster, the lobster's claw is often full and fleshy, not small. Lobsters have a cephalothorax which is the place where their head connects to the thorax. The carapace covers the cephalothorax and the thorax. The carapace is made of chitin. A lobster's head has two pairs of antennae and three pairs of jaws. Lobsters use their antennae to feel where they are. They cannot see very well. The abdomen of the lobster includes legs and a tail fan.

Lobsters usually move slowly by walking on the bottom of the sea floor. However, when they are in danger and need to escape, they swim backwards quickly by curling and uncurling their abdomen. Lobsters can move at a speed of 5 metres per second.